As a conventional example of a cooling medium supply/discharge device which cools a superconducting coil of a superconducting rotary device to a temperature at which the superconducting coil is retained in a superconductive state, there is disclosed a structure which is provided between a rotor of the superconducting rotary device and a refrigerating device as a supply source of a cooling medium (refrigerant) with an extremely low temperature, such as helium gas, to supply the cooling medium to the superconducting rotary device and discharge the cooling medium from the superconducting rotary device (e.g., see Patent Literature 1, 2).
This cooling medium supply/discharge device includes an inlet cooling tube and an outlet cooling tube which are coaxial with the rotary shaft of the rotor of the superconducting rotary device. The outlet cooling tube has a larger bore diameter than the inlet cooling tube. The inlet cooling tube is provided inside the outlet cooling tube. The inlet cooling tube is configured to introduce the cooling medium from the refrigerating device into the rotary shaft of the superconducting rotary device. The outlet cooling tube is configured to return to the refrigerating device the cooling medium discharged from the rotary shaft of the superconducting rotary device, in a location which is outward relative to the inlet cooling tube. The inlet cooling tube and the outlet cooling tube are fixed to the rotary shaft of the superconducting rotary device, and configured as rotary tubes, respectively, which rotate according to the rotation of the rotary shaft.
In the structure disclosed in Patent Literature 1, it is necessary to rotate the rotary shaft of the superconducting rotary device with respect to the refrigerating device. Therefore, a stationary tube fixed to the refrigerating device or other part which is in the vicinity of the refrigerating device, and a rotary tube are slidable via a seal member to prevent a leakage of the cooling medium from a coupling portion of the stationary tube and the rotary tube. However, if heat is generated by the sliding operation of the stationary tube and the rotary tube, the temperature of the cooling medium is increased, or the seal member is degraded, and thereby the life is shortened. To avoid this, in the structure disclosed in Patent Literature 2, a small gap is formed in a portion of the stationary tube and a portion of the rotary tube which portions overlap with each other in such a way that they do not slide with respect to each other, and a recess is provided on the outer peripheral surface of one of the stationary tube and the rotary tube, which has a smaller diameter, thereby controlling the leak amount of the cooling medium in the coupling portion.